Heat Index Calculator
Calculate the heat index — how hot it feels once humidity is accounted for — with the official NWS regression and risk categories.
How to use this calculator
- 1Enter the air temperature and relative humidity.
- 2Read the risk category — it is more actionable than the temperature itself.
- 3Add several degrees mentally if you will be in direct sun; the index assumes shade.
How the calculation works
HI = −42.379 + 2.04901523·T + 10.14333127·R − 0.22475541·T·R
− 0.00683783·T² − 0.05481717·R² + 0.00122874·T²·R
+ 0.00085282·T·R² − 0.00000199·T²·R²- HI
- Heat index in °F
- T
- Air temperature in °F
- R
- Relative humidity as a percentage
This is the Rothfusz regression used by the US National Weather Service — a curve fitted to Steadman’s 1979 heat-balance model rather than derived from physics directly.
Two documented corrections apply at the edges: below 13% humidity between 80–112 °F, and above 85% humidity between 80–87 °F.
Below about 80 °F the regression breaks down, so a simpler approximation is used instead. Metric inputs are converted to Fahrenheit internally because the coefficients are defined there.
Worked example
32 °C at 70% humidity
- 1.32 °C is 89.6 °F, which is above the 80 °F threshold, so the full regression applies.
- 2.Substituting T = 89.6 and R = 70 into the nine-term polynomial gives 104.74 °F.
- 3.Converting back: 104.74 °F is 40.41 °C.
- 4.That is 8.4 °C hotter than the air temperature, and lands in the "Danger" band.
Result: 40.4 °C — Danger category
What heat index actually measures
Heat index is an estimate of how hot the air feels to the human body once humidity is factored in, expressed in the same degrees as a thermometer reading so it can be compared directly against the actual air temperature. It is sometimes called "apparent temperature," which is a more accurate description than "feels like" — the number is not a subjective impression but a calculated estimate of the combined load that heat and humidity together place on the body.
Humidity belongs in the calculation because the body’s main defence against overheating is evaporating sweat, and evaporation is a humidity-dependent process — the same air temperature can range from tolerable to dangerous depending purely on how much more moisture the air can still absorb.
Why humidity changes how heat feels
Sweat cools the body only when it evaporates — the change from liquid to vapour is what carries heat away from the skin, not the presence of moisture on it. In dry air, sweat evaporates quickly and the cooling effect is strong even at fairly high temperatures. As humidity climbs, the air is already closer to holding as much water vapour as it can, so sweat evaporates more slowly, the body’s main cooling channel weakens, and the same air temperature starts to feel — and physiologically behave — like a hotter one.
That is also why heat index rises so sharply once both temperature and humidity are high together: the two effects compound rather than simply add, which is exactly what the formula’s cross-terms between temperature and humidity are built to capture.
From a physiological model to a formula a forecaster can run in seconds
The heat index used today traces back to a detailed 1979 heat-balance model built by physicist Robert Steadman, which modelled how a human body actually exchanges heat with its surroundings — far too complex a calculation to run for every forecast, every hour, across a whole country. Meteorologist Lans Rothfusz condensed that model into a polynomial regression that approximates Steadman’s results closely enough for daily forecasting use, and the US National Weather Service adopted it in 1990 as the standard heat index formula, with a simpler approximation swapped in below about 80 °F, where the full regression is not needed and does not perform reliably anyway.
Why the heat index matters beyond a summer forecast graphic
Heat is a genuine hazard, and the index exists to turn two separate numbers — temperature and humidity — into a single, actionable risk signal.
- Workplace heat safety — guidance for outdoor and physically demanding indoor jobs increasingly sets rest-break and hydration schedules around heat index thresholds rather than temperature alone.
- School and youth sports — athletic associations commonly use heat index bands to decide when practices move indoors, get shortened, or add mandatory water breaks.
- Protecting at-risk groups — older adults, young children, and people with certain medical conditions or medications lose heat-regulating ability sooner than a healthy adult, so a heat index that is merely uncomfortable for one person can be dangerous for another.
- Event and outdoor recreation planning — race organisers, festivals and summer camps often publish their own heat index cutoffs for cancelling or modifying outdoor activity, the same logic school sports districts use.
What this assumes, and where it stops
Assumptions
- Shade with a light breeze. Direct sun adds up to 8 °C.
- A healthy adult of average build, not acclimatised to extreme heat.
Limitations
- The regression is unreliable below about 27 °C, where humidity has little bearing on comfort.
- It does not account for wind speed, solar radiation, clothing or activity level.
- Canada uses Humidex and Australia its own apparent temperature; both give different figures for identical conditions.
- Individual heat tolerance varies enormously with age, fitness, medication and acclimatisation.
Common questions
Why does humidity make heat feel worse?
Because your body cools itself by evaporating sweat, and evaporation slows as the air approaches saturation. At 90% humidity the sweat still forms but barely evaporates, so it carries almost no heat away. That is why 32 °C in a dry climate is tolerable and 32 °C in a humid one can be dangerous.
What heat index is dangerous?
Above about 39 °C (103 °F) the NWS classifies conditions as "Danger" — heat exhaustion becomes likely and heat stroke is possible with prolonged exposure. Above 52 °C is "Extreme danger". But these are population guidelines; older adults, young children and people on certain medications are at risk well below those thresholds.
Sources
- Heat index equation and chart — US National Weather Service
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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